A vehicular display device constituting a vehicular display system is disclosed, for example, in Japanese Patent Application Laid-Open Publication (JP-A) No. 2011-111123, which displays a first image (containing information of a vehicle speed and warnings, for example) on an HUD (head-up display) and a second image (containing information of a speed meter and a tachometer, for example) on a meter by using a single light source unit such as a light-projector.
In the vehicular display device disclosed in JP 2011-111123A, however, an ECU (electronic control unit) for controlling the light source unit must include a luminance value adjustment means which controls the luminance of the light source unit based on the higher illuminance of a cabin-outside illuminance and a cabin-inside illuminance detected respectively by an external illuminometer and an internal illuminometer, and a pixel value adjustment means which performs, based on the lower illuminance of the detected cabin-outside illuminance and the detected cabin-inside illuminance, control to reduce the pixel value of one of the first image and the second image to be lower than the pixel value of the other. In other words, the ECU must have a complex configuration, and the manufacturing cost of the vehicular display device is inevitably increased.
To solve this problem, an improved vehicular display system has been proposed by the present inventors which can reduce the manufacturing cost of the vehicular display device by using a mobile device (such as a tablet device) having a mobile display part, instead of the HUD. In the improved vehicular display system, however, a driver might be unable to see part of a display screen depending on a position in which the mobile device is disposed. Not only in the improved vehicular display system, this kind of problem would occur when the mobile device is used together with a vehicle on-board unit in a vehicle.